Process for fluorescence detection of extremely small flaws



June 4,v 1968 MENHCUS METHOD SPOT DIAMETER mm.

J; R. ALBURGER 3,386,920

PROCESS FOR FLUORESCENCE DETECTION OF EXTREMELY SMALL FLAWS Filed Oct. 4, 1965 sinsmvrrvmou (1,) -onovl cauclurnanoufu uuwspl. Gus/nur) INVENTOR.

United States Patent O 3,386,920 PROCESS FOR FLUORESCENCE DETECTION F EXTREMELY SMALL FLAWS James R. Alburger, 5007 Hillard Ave., La Canada, Calif. 91011 Continuation-impart of application Ser. No. 323,529, Nov. 13, 1963. This application Oct. 4, 1965, Ser. No. 492,674

23 Claims. (Cl. 252-30L2) The present application is a continuation-in-part of my copending application, Ser. No. 323,529, filed Nov. 13, 1963, for Fluorescent Tracers, which latter application was a continuation-in-part of my application, Ser. No. 149,061, filed Oct. 31, 1961, for Fluorescent Tracers, which last application was a continuation-in-part of my application, Ser. No. 82,374, filed I an. 13, 1961, for Fluorescent Penetrant Tracers, all now abandoned.

The present invention relates to fluorescent tracers, and more particularly to fluorescent tracers having improved sensitivity performance characteristics.

Fluorescent tracers are well known in the prior art, and have usually been comprised of one or more fluorescent dyes suspended in a suitable carrier material. These tracers have found advantageous usage in industrial inspection processes. Thus, such tracers have been employed in the detection of surface flaws in parts constructed of metal, ceramic, or other material. When used for this purpose, the fluorescent dye and carrier is utilized in the form of a penetrant liquid which forms entrapments in the flaws and renders the latter more readily detectable than might be the case with ordinary visible colored dyes.

In the use of fluorescent penetrants for detection of extremely small flaws, it has heretofore been thought that the ability of the tracer to detect the flaws is a function of its fluorescent brightness. Basically, fluorescent brightness, of course, depends upon the particular fluorescent dye or dyes used. However, such brightness can be enhanced by the well known effect of cascading of fluorescence. Whereas the brightness effects produced by combinations of two or more fluorescent dyes are ordinarily approximately additive, cascading involves the transfer of radiant energy from one dye component to another, with a consequent increase in brightness which is more than merely additive.

Since, as has been pointed out, the sensitivity of a fluorescent substance as regards its ability to reveal its presence in micro-traces has been equated directly with lfluorescent brightness, it has often been the practice in industrial inspection processes to attempt to maximize such sensitivity of the fluorescent tracer used by increasing thek fluorescent brightness thereof; e.g., through the use of the aforementioned effect of cascading of fluorescence. Thus, when a flaw and its tracer entrapment are so small as to be virtually invisible under black light, it has usually been felt necessary to employ a tracer of increased fluorescent brightness so as to make the flaw detectable. However, in spite of the attempts heretofore made to maximize the sensitivity of fluorescent penetrant materials by increasing the fluorescent brightness thereof, presently known fluorescent penetrant tracers are unable to detect extremely small flaws which may result from such effects as inter-crystalline corrosion or creep cracks, and which may have dimensional magnitudes on the order of *5 to 10r6 centimeters.

The failure in the prior art to produce fluorescent tracers which are able to detect flaws of such small dimensional magnitude has resulted primarily from the emphasis which has been laid on the fluorescent brightness of the particular dyes employed, as discussed above. The possibility of improving fluorescent response through increase in concentration of the dye used has not been ice given consideration. In fact, the prevailing belief is that i any substantial increase in dye concentration is contraindicated. Such belief is based on the generally accepted assumption that fluorescent brightness response tends to be self-quenching when the fluorescent dye is used in a concentration in solution above an approximate range of about 0.5% to 2% by weight.

In addition to the aforementioned shortcomings of known fluorescent dyes when used for certain applications in the penetrant inspection field, such dyes have also been unable to serve adequately for other detection or testing purposes, or for marking, decoration, and other applications in which thin-film characteristics are desired.

The principal object of the invention, therefore, is to provide improved fluorescent tracer materials.

Another object of the present invention is to provide fluorescent tracer materials having improved sensitivity performance characteristics.

Still another object of the present invention is to provide fluorescent tracers which exhibit sensitivity levels far greater than that obtaintable through enhancement of fluorescent brightness.

A further object of the present invention is to provide fluorescent tracers having improved characteristics for testing, marking, decorative, and other purposes in which thin-film properties are desirable.

A still further object of the invention is to provide fluorescent tracers for penetrant inspection purposes, capable of detecting flaws having dimensional magnitudes within a range as low as 10-5 to 10-6 cm. or smaller.

These and other objects of the invention will become more apparent from the following description thereof when read in conjunction with the accompanying drawings, in which:

FIG. 1 is a chart employable in conjunction with the use of the fluorescent tracers of the invention, a reading of said chart for a particular tracer being shown;

FIG. 2 is a diagrammatic representation of a surface flaw illustrating the use of tracer materials of the invention;

FIG. 3 is a diagrammatic representation of a leakage flaw illustrating the use of tracer materials of the invention; and

FIG. 4 is a diagrammatic representation of a thin film surface coating illustrating the use of tracer materials of the invention.

The present invention provides fluorescent tracers, each of which is formulated essentially by the solution in a suitable solvent of a fluorescent sensitizer selected from a group to be defined hereinbelow, and at least to a minimum level of concentration up to a maximum level of concentration depending on the solubility `limit of the sensitizer, as will be described. In order to understand the reason for the particular grouping of sensitizers made, and for the set minimum level of concentration, reference should first be made to the concept, which I have formulated, of thin-film fluorescence. In accordance with this concept, a fluorescent sensitizer, such as a fluorescent dye or other fluorescent substance, when in solution exhibits the characteristic of a threshold of film thickne-ss below which fluorescence response ceases. Thus, for a given fluorescent dye dissolved in a particular carrier at a particular concentration, there exists a specific film thickness below which there is no fluorescence, and above which there is a fluorescence response. The threshold thickness of the tracer film may be termed the dimensional sensitivity of the fluorescent tracer. The concept of thin-film fluorescence is applicable to a fluorescent substance in solution in a solid form (e.g., as a plastic or resinous material) as well as in liquid form.

As part of my discovery, it has been found that the dimensional -sensitivity of a fluorescent sensitizer is independent of the color or lluorescent brightness thereof. Instead, each luorescent sensitizer substance can be assigned a specific sensitivity value k, as will be described hereinbelow, and the dimensional sensitivity is a function of k and the concentration C of the sensitizer. The dimensional sensitivity of a given fluorescent sensitizer can, over a useful range of concentrations, be determined by a method to be described below. In conjunction with such method, there may 'be mea-sured, for each fluorescent material in solution, a sensitivity index Is value, which is defined in accordance with the following relationship:

The sensitivity index Is value of a fluorescent material in solution is a direction function of the threshold lm thickness of said material, and serves as a convenient means of expressing the dimensional sensitivity thereof.

For the purpose of measuring the dimensional sensitivity of a luorescent material in solution, and also of determining the Is value thereof, I employ a Meniscus Method, which is described in my U.S. Patent No. 3,107,298, granted Oct. 15, 1963, for Apparatus for the Measurement of Fluorescent Tracer Sensitivity. In practicing the invention disclosed in the latter patent, a ilat glass platen is positioned under a black light, and a drop of liquid having dissolved therein a fluorescent substance is placed on the platen. A convex lens having a preferred radius of curvature of 106 cm. is then placed over the drop of liquid, and so as to rest on the platen. At the point of contact between the lens and the platen, the liquid lilrn has :a thickness of substantially zero, and a meniscusshaped iilm surrounds said contact point.

As pointed out in Patent No. 3,107,298, the thickness of the liquid film varies continuously with the radial distance from the contact point. However, the fluorescence response as seen under black light is a distinctly discontinuous function, so that a noniluorescent spot is seen in the region of the contact point. If the llat platen is made of a black glass, the nonfluorescent spot appears as a black spot which contrasts sharply with the surrounding area of lfluorescence, and which can be measured as to its diameter with good accuracy. The diameter of this black spot is used as a measure of the lilm thickness above with lluorescence response occurs, and below which iluorescence ceases. For a given lluorescent sensitizer dissolved in a particular solvent material, the diameter of the black spot varies depending upon the concentration of the iluorescent sensitizer.

Inasmuch as the transition of fluorescence response, with respect to lm thickness, is really a continuous function, the toe of the transition curve approaches zero response as the film thickness is made smaller, but, theoretically at least, never actually reaches zero. As a practical matter in locating the point corresponding to the so-called dimensional threshold of fluorescence, a point on the transition characteristic curve of the tracer is taken at a film thickness which is one-tenth (3l/10) the iilm thickness at the mid-point of the transition curve, Where the brightness of fluorescence response falls to a value below about 2% to 7% of the maximum brightness which appears in a relatively thick film of the tracer composition. In the above-mentioned Meniscus Method test, the diameter of the black spot would be taken as the distance between the points where brightness response begins to rise fairly steeply, at the 2% to 7% values of maximum brightness as indicated above.

In measuring the fluorescent characteristics of various materials, employing the above-described Meniscus Method, I have discovered the iluoranthene (ClsHm) exhibits uorescent characteristcs which render it useful in the establishment of a reference standard, against which the performance of a fluorescent material may be evaluated. Referring, now, to FIG. 1, a five-cycle single logarithmic chart is shown having a logarithmic horizontal or abscissa, scale representing Is values, ranging from .0l to 1000. The vertical, or ordinate, scale at the left represents spot diameters as determined by the above-described Meniscus Method of measurement, and ranges from 0 to 4 mm. A straight line 5 is drawn as shown, between two points having respectively an ordinate of 4 and an abscissa of .01, and an ordinate of 0 and an abscissa of 1000. Said straight line provides a suitable reference characteristic for purposes of standardization, which reference characteristic corresponds to an imaginary fluorescent substance having a specic sensitivity k of unity l.

Since the lluorescent properties of fluoranthene are closely approximated by those represented by the diagonal line 5, it is often convenient to -use fluoranthene as a reference material in evaluating the threshold response characteristics of other sensitizer materials. In such usage, a series of tracer solutions of iluoranthene may be prepared having accurately measured concentrations of lluoranthene in a suitable solvent carrier such as N-methyl-Z- pyrrolidone, as for Example 2, 5, 10, 15, 25, 40, 75, 100, 150, 200, etc., grams per pint. N-methyl-Z-pyrrolidone is mentioned as a suitable solvent for fluoranthene since it will dissolve this sensitizer to concentrations greater than 200 grams per pint at room temperature. The unit of measure, grams per pint, is used for the reason that such values are numerically equal to pounds -per 55 gallons, so that laboratory test results may be quickly and conveniently translated for production of drum quantities of material.

If a series of lens and platen setups are prepared side-byside under black light using the above-described iluoranthene solutions, they will exhibit a graduated scale of spot diameters ranging from 3 or more down to as small as .5 mm. or less. If, then, a test solution of a given fluorescent sensitizer is prepared in a suitable solvent carrier and to a certain concentration, for example, l5 grams per pint, a Meniscus Method test of such solution will provide a characteristic spot diameter which may be quickly estimated by a visual comparison with the iluoranthene scale. For example, if the test material, at a concentration of l5 grams per pint yields a spot diameter similar to iluoranthene at 40 grams per pint, then the Is value of the material is 40 and its k value is 40;-l5=2.66. By use of the above-outlined procedure, it has been possible to rapidly evaluate the iluorescence response characteristics of the many existing iluorescent dyes and other chemical materials so as to select those which are useful as sensitizers for the purpose of this invention, and which are listed hereinbelow. l

It should be mentioned that fluorescence response characteristics, and likewise the k value for a given sensitizer material, may vary depending on the solvent carrier which is employed and the concentration level, so that in cases where precision in measurement is wanted, it is desirable to specify the conditions under which the measurements are made. For most practical purposes, adequate accuracy may be obtained by determining the k value for a sensitizer at a concentration of 15 grams per pint using a solvent carrier which has suicient solvent capability for the sensitizer such that the concentration of l5 grams per pint is well below the saturation point of the solution. A wide variety of solvent materials may .be employed, as will be described below.

At the right hand side of the chart of FIG. 1, an ordinate scale, representing dimensional sensitivity (threshold iilm thickness) in millimicrons, is provided. In view of the geometry of the standard testing apparatus employed, and particularly in view of the known radius of curvature of the lens, the dimensional sensitivity (or threshold lm thickness) is mathematically calculated, being a direct function of the spot diameter.

As will be clear, the dimensional sensitivity of a iiuorescent dye solution is a basic physical characteristic thereof, and the Meniscus Method represents a convenient means for the determination of the lvalue for this characteristic. It should be mentioned that nonlluorescent Ivisible color dyes in solution also exhibit thin lilm dimensional thresholds similar to the behaviour of fluorescent substances. However, nonfluorescent dyes rarely if ever provide dimensional threshold properties smaller than 250 millimicrons, and their k values are virtually always less than about .1. Accordingly, fluorescent materials, and particularly those which have k values above about .2, and which are thus useful for the purpose of this invention, are in a category distinct from ordinary nonfluorescent visible color dyes or coloring materials or ordinary low-sensitivity fluorescent dyes.

It will thus be understood that the dimensional sensitivity, or threshold film thickness, of a fluorescent material is a function -of the specific sensitivity k of the sensitizer dye or other fluorescent substance employed, and of the concentration `C of the latter. The important significance of the foregoing discussion with regard to dimensional sensitivity may be best understood by referring now to FIG. 2 which illustrates the behaviour of a fluorescent tracer of the invention as used in the well-known inspection penetrant process. A surface 7 has a small flaw 8 in the form of a crack, a porosity, or an intercrystalline separation. A liquid tracer, containing a fluorescent dye, is applied to surface 7 such that it enters the flaw t8 and forms an entrapment 9. The surface 7 is cleaned to remove residual penetrant tracer, after which the liquid entrapment exudes from the flaw 8 to form a micro-thin film 10 of liquid in the region of the surface flaw.

It will be understood that the amount of liquid which can exude from the flaw 8 depends on the size of the flaw, and likewise the thickness of the exuded tracer film will depend on the size of the surface flaw 8. If it is assumed that the thickness of the exuded film 10 is dimensionally of the same order of magnitude as the size of the surface flaw 8, then it follows that for an inspection penetrant to be capable of revealing the presence of flaws having dimensions of the order of a half wavelength of light or 250 millimicrons, the dye tracer rnust have a dimensional threshold smaller than 250 millimicrons, and an Is value greater than about as shown on the chart of FIG. 1.

Referring now to FIG. 3, which illustrates the behaviour of a liquid tracer of the invention as used in a leak detection process, a wall 12 of a tank or pipeline may be tested for liquid leaks through a microscopic leakage path 13 by applying a dyed liquid tracer 14 on one side of the wall 12 and allowing the dyed tracer to migrate through the leakage path 13, either by capillary action or by pressurizing the liquid. As the liquid tracer exudes from the leakage path 13, it may yield a micro-thin film 15 surrounding the point of leakage.

In cases where a leakage condition is extremely small, it may require quite a long time for a substantial thickness of the tracer film 15 to be generated. Thus, the leak detection capability of a dyed liquid leak tracer may be equated with the thin-film indication response of the leak tracer. It will be understood that a fluorescent leak tracer, such as one which will yield a fluorescence response at lm thicknesses of the order of 250 millimicrons as provided by the tracer liquids of this invention will be effective in the detection of micro-leaks which will be undetectable with solutions of ordinary nonfluorescent dyes or ordinary low-sensitivity fluorescent dyes.

Referring now to FIG. 4, a surface 17 is coated with a thin film of plastic material 18 which is to act as an electrical insulating layer. For the purpose of this illustration, it is desired that the thickness of the applied insulating layer shall be smaller than a half wavelength of light or 250 millimicrons. At the same time, it may be desired that the presence of this thin insulating layer shall be revealed by its fluorescent response. It will be seen, therefore, that in order for such a layer to yield a fluorescence response, the Is value of the tracer material, of which the film consists, must be greater than about 15, as shown on the chart of FIG. l.

Again referring to FIG. 4, the layer 18 may be formed by means of a waxy film from a crayon, or by means f' chemical processes, fluorescence tagged soldering fluxes or chemical process materials may form layers of unwanted residues which are desired to be detected by means of their fluorescence response. In any event, and where the applied film is to exhibit fluorescence at film thicknesses smallerl than 250 millimicrons, the Is value must be greater than about l5 as shown on the chart of FIG. 1.

Solutions of the fluorescent substances of this invention all exhibit thin-film fluorescence response thresholds at various concentrations corresponding approximately to those represented by the diagonal line in FIG. l. Inasmuch as an IS value of 15 corresponds to a dye concentration of about l5 grams per pint, a class of particularly useful fluorescent tracers (i.e., those which yield fluorescence response at film thicknesses smaller than 250 millimicrons) may be defined as those which contain a fluorescent substance, as specified hereinbelow, within a range of proportional concentrations greater than at least about l5 grams per pint up to the limit of solubility Of the fluorescent substance.

The sensitizers employed in accordance with the invention are selected so that each has a comparatively high k value, i.e., within the approximate range of .2 to 3. Certain of the sensitizers are identified below in accordance with their standard designations in the Color Index (2nd ed., 1956, vols. l to 4), published by the Society of Dyers and Colourists, Dean House, Picadilly-Bradford, Yorkshire, England; the American Association of Textile Chemists and Coloriste; and Lowell Technological Institute, Lowell, Mass., U.S.A. The k value, as determined by the comparison method described above, is indicated for each sensitizer thus listed.

For commercial usage, the Color Index listing provides an accurate designation for a given fluorescent dye. Certain other fluorescent materials, which are not normally considered to be dyestuffs, may be designated according to their appropriate chemical structures, as illustrated hereinbelow, or by their commonly employed chemical names. The many useful fluorescent dyes Which are listed in the Color Index, and others which are not so listed, are well documented in the patent literature. It should be understood that not all fluorescent dyes are necessarily useful for the purpose of the invention. This is because certain fluorescent dyes may lack adequate solubility, or their k values may be excessively low. Substances which are useful for the purpose of this invention may be listed in groupings as follows. Color Index designations are given where applicable.

(a) Fluorescent (d)amn0stlbine(d)sulfonic acid dyes.-Most of these dyes are characterized by k values in the range of from .2 to 3.

(b) Fluorescent dbenzothophene dyes.-Most of these dyes are characterized by k values in the range of from .2 to 3.

(c) Fluorescent monoazole dyes-These materials are characterized by k values in the range of from .2 to 3.

(d) Fluorescent bsazole dyes.-These materials are characterized by k values in the range of from .2 to 3.

(e) Fluorescent coumarz'n dyes.--These materials are characterized by k values in the range of from .2 to 3, usually above 1.

(f) Fluorescent perylene dyes-These materials are characterized by k values in the range of .2 to 3, usually around .25.

y (g) Fluorescent naphthalic acid mide dyes-These materials are characterized by k values in the range of from .2 to 3.

(h) Fluorescent pyrdotrazole dyes.-These materials are characterized by k values in the range from .2 to 3.

(i) Fluorescent di-hydrocolldz'ne dyes-These materials are characterized by k values in the ran-ge of from .2 to 3, usually around .25.

(j) Fluorescent acrz'dne dyes- These :materials are characterized by k values in the range of from .2 to 3.

They are listed in the Color Index under the following Color Index (C.I.) designations.

C.I. Number:

C.I. Basic Orange 14, 15, 16, and 23. C.I. Basic Yellow 6 and 9.

(k) Fluorescent xanthene dyes-These materials are characterized by k values in the range of lfrom .2 to 3, usually around .25. They are listed in the Color Index under the following Color Index designations.

C.I. Number:

C.I. Basic Red 8. C.I. Basic Violet l1.

(l) Fluorescent brightenn-g agent dyes.-These matcrials are listed in vol. 4 of the Color Index under Fluorescent Brightenin-g Agents (for textile usage). The C.I. B/A (Brightening Agent) numbers and applicable k values are indicated as follows:

C.I. B/A 2, k=.25 C.I. B/A 5, k=.2

C.I. B/A 54, k=l.35 C.I. B/A 57, k=l.35

C.I. B/A 9, k=1.35 C.I. B/A 25, k=.25 C.I. B/A 30, k=.2 C.I. B/A 53, k=1.35 C.I. B/A 56, k=.2 C.I. B/A 60, k=l.0 C.I. B/A 66, k=.25 C.I. B/A 69, k=1.0 C.I. B/ A 74, k=.5 C.I. B/A 77, k=.5 C.I. B/A 3, k=1.0 C.I. B/A 6, k=.25 C.I. B/A 22, k=.23 C.I. B/A 26, k=1.`7 C.I. B/A 46, k=2.3

C.I. B/A 6l, k=.65 C.I. B/A 67, k=1.0 C.I. B/A 70, k= 1.7 C.I. B/A 75, k=.5 C.I. B/A 78, k=2.65 C.I. B/A 4, k=.8 C.I. B/A 8v, k=.5 C.I. B/A 24, k=.2 C.I. B/A 29, k=.5 C.I. B/A 47, k=.65 C.I. B/A 55, k=1.7 C.I. B/A 59, k=.25 C.I. B/A 65, k=.2 C.I. B/A 68, k=l.7 C.I. B/A 71, k=1.0

C.I. B/A 76, k=l.35

(m) Miscellaneous fluorescent dyes- These materials are listed in the Color Index under various dye classifications. The particular dyes and their k values are indicated as follows:

(n) Fluorescent chemical compounds-The following materials are characterized by k values in the range of from .25 to 1.7:

Fluoranthene-ClsHm, represented by the structural formula:

Pyrene-ClHlo, represented by the structural formula:

In the formulation of the iluorescent tracers of the invention, color-formers may be employed as well as fluorescent sensitizers. A colorforrner is an ordinary tluorescent or nonlluorescent dye material which has the ability to contribute color to or shift the lluorescent color of a dye mixture containing at least one sensitizer. A colonformer, in other words, provides a characteristic color for the tracer. While the sensitizers of the invention have k values within the approximate range of .2 to 3, color-formers or ordinary dyes usually have k values somewhat less than .l (even though they may yield intense coloration or high liuorescent brightness).

Many dyes are suitable for use as coloreformers. These ordinary dyes or coloring materials, which are also listed in the Color Index, may be used in conjunction with any of the above-listed sensitizers, the only requirement being that the color-former must be selected for its solubility in the same solvent carrier in which the particular sensitizer is dissolved, as well as for its color characteristic.

Most of the sensitizers which are grouped above, fluoresce blue, bluish white, or bluish green, while a few lluoresce yellow, orange, or red. It has been found that only in a comparatively few instances will a liuorescent dye which is able to provide a substantially high dimensional sensitivity produce some color other than blue or green.

The sensitizers 'grouped above may be employed either singly, or in combination with one another. A prerequisite for combination of the sensitizers, of course, is that those combined shall each be compatible with a particular solvent system.

-A wide variety of solvents may be utilized as carriers for the above-designated fluorescent sensitizers. The following listing in Table I includes solvent materials, both liquid and resinous (or plastic), which have been tested with the foregoing fluorescent sensitizers. In all cases, it has been `found possible to obtain a desired dimensional threshold in the dye solution smaller than 250 millimicrons.

Table I.-S0lvent materials tested water acidirfed water alkaline water ethylene glycol diethylene glycol triethylene glycol polyethylene glycols (M.W.-200 to 6000) ethylene glycol monobutyl ether diethylene glycol monobutyl ether ethylene glycol monoethyl ether diethylene glycol monoethyl ether polyoxyalkaline glycols '(Ucon uids) dimethyl formamide N-rnethyl-JZ-pyrrolidone dimethyl solfoxide N-vinyl-pyrrolidone silicone oil (Corning 200) nitroethane polyester resin (Am. Cyanamid Laminac) alkyd resin (Amer. Cyan. Rezyl) TABLE l-Continued rosin methanol ethanol isopropanol isodecanol acetone me-thyl ethyl ketone methyl isobutyl ketone diace-tone alcohol mesityl oxide isophorone ethyl amyl ketone mineral thinner (naphtha) kerosene carbon tetrachloride perchloroethylene methylene chloride fiuorocarbon oil (Hooker Fluorolube) sodium petroleum sulfonate urea formaldehyde resin (Am. Cyan. Bee-tlc) epoxy resin (Shell Epon) vinyl resin (Union Carbide VYGH) IIn addition to the above-listed solvents, it has been found -that the solvent carriers listed in my copending application, Ser. No. 492,676, filed Oct. 4, 1965, now issued Patent No. 3,3'11,479, for Penetrant Inspection Process and Compositions, now abandoned, also function well as carriers for the fluorescent sensitizers of this invention.

The choice of a solvent system for use in conjunction With a particular sensitizer of the invention, and the manner of effecting the solution of the sensi-tizer are accomplished in accordance with procedures well known in the art. Thus, for example, a sensitizer which is normally relatively insoluble in mineral spirits may be coupled into solution `with the lat-ter by means of a glycol ether. Likewise," a sensitizer which is relatively insoluble in Water may be coupled int-o a water solution by means of an alcohol additive or by use of a surfactant, such as ethoxylated alkylphenol. Although aliphatic mineral solvents have relatively low solvent capabilities for most of the sensitizers of the invention, the solution may still often be effected through the additional use of a small amount of aromatic mineral solvent, the latter materially increasing the ability of the liquid mixture to carry the sensitizer into solution. K

It has been dis-covered that fluorescent materia-ls produced in the manner described above may exhibit dimensional thresholds of fluorescence of the order of 2.5 X-5 to 2x10*6 cm. or less, if there is employed a proportional concentration :of sensitizer of at least about 115 grams per pint. `Referring again to FIG. l, i-t will be noted tha-t the value of 2.5 105 cm., or 250 millirnicrons, is equivalent to a sensitivity index Is value of 15. Thus, it will be understood -that such an Is value of (within abou-t one order of magnitude), or greater, is achieved upon the dissolution in a suitable solvent of about 15 grams per pin-t, or more, of any of the sensitizers of the group specified.

I-t will be understood that the dissolution of a sensitizer having a k value of .2 in the proportional concentration of 15 grams per pint will yield a sensitivity index Is value of 3, which corresponds to a dimensional threshold of fluorescence of about 500 millimicrons. This sensi-tivity condition is well Within one order of magnitude of a desired sensitivity threshold characteristic of 250 millimicr-ons, and such materials are, therefore, considered to be useful for the purpose of the invention. F-or sensitizers which have k values as low as .2, it is merely necessary to employ a proportional concentration of about 75 grams per pint in order to achieve a desired dimensional sensitivity of 250 millimicrons.

Some of the sensitizers have exceptionally high solubilities in particular solvents, and may permit use at c-oncentrations as high as about 300 grams per pint. Thus, a sensitizer having a k value of one (1) will, when used in a concentration of about 15 grams per pint, provide approximately a value of Is=kC=15, with equivalent dimensional sensitivity of the order of 2.5 105 cm. 'HOW- ever, if the sensitizer is fluoranthene, and the solvent is N-methyl-Z-pyrrolidone, the concentration may 'be increased up to a value of the order of 300 grams per pint, with a corresponding increase in dimensional sensitivity to a value of the order of 2x106 cm. or less. Certain luorescent coumarin dyes, notably C.I. Fluorescent Brightening Agents 68 and 69, also have excellen-t solubility in solvents such as N-methyl-Z-pyrrolidone, dimethyl formamide, or dimethyl sulfoxide, and are thus capable of providing dimensional sensitivities in the range of 2x10-6 cm. of less.

It may be noted that the minimum concentration of about `15 grams per pint, employed in accordance with the invention, is equivalent to about 333% concentra-tion by weight. This value is well above the maximum values for the concentrations of fluorescent `dyes employed in the prior art, which, as indicated above, have been of the order of .5% to 2%. The maximum allowable concentration depends only on the solubility of the particular sensitizer in a given solvent carrier. All of the above listed sensitizers will dissolve in appropriate solvents to concentrations of the order of 40 grams per pint or about 8% by Weight, While a large number of the sensitizers will dissolve in suitable solvents to concentrations of the order of grams per pint or about 20% by weight. A few of the sensitizers may be dissolved in appropriate solvents t-o concentrations greater than about 300 grams per pint, or about 60% by Weight or more.

It should be pointed out that, although the limit of dimensional sensitivity for most solutions of sensitizers is about 2 10 to 4 106 cm., it is possible to extend the dimensional sensitivity of a given fluorescent tracer Well down toward 10-r1 cm. by means of a liquid developer technique which is the subject of my now issued U.S. Patent No. 3,184,596, granted May 18, 1965, for Flaw Detection Method Using a Liquid Solvent Developer.

Primarily, as a result of the very low threshold film thickness exhibited by the fluorescent tracers of the invention, the use of said tracers offers advantages not heretofore obtainable in industrial inspection, production control, quality assurance, and other similar applications. For example, in the case of jet engine turbine blades, micro-cracks often occur which have dimensions on the order of 10-5 cm. In the case of heat resistance surface coatings for space vehicles, porosity defects may occur which likewise have dimensions on the order of 10*5 cm.

When employing a fluorescent penetrant tracer for flaw detection purposes, the process in its simplest form includes dipping the part to be tested in the penetrant, draining the part, cleaning the surface penetrant by washing (and in the case of non-washable penetrant, applying an' emulsiiier-coupler dip prior to washing), and finally inspecting the part under black light. In some cases, an additional step of applying a so-called developer is ernployed for the purpose of enhancing the fluorescent brightness of the flaw indications.

EXAMPLE NO. 1

As an example of the use of the Meniscus Method in connection with the chart of FIG. 1, a particular fluorescent substance (pyrene) present in a liquid solvent (ethylene glycol monethyl ether) at a concentration of 9 grams per pint exhibited a spot diameter of 1.45 mm. The equivalent sensitivity index Is value corresponding to this spot diameter was determined by reading on the chart across to the reference line 5, along line x, and was found to be 15 at line y. The specific sensitivity k of the substance (at this concentration) was then determined in accordance with the relationship IS=kC. Thus, k=15 divided by 9, or 1.7. The equivalent dimensional senll sitivity, or threshold film thickness, was also read from the chart by following line x across to the right hand scale of ordinates, and was found to be equal to 250 millimicrons.

EXAMPLE NO. 2

A penetrant tracer having the following formulation was prepared:

Ethylene glycol monobutyl ether gal. Fluoranthene lbs. C I. Brightening Agent 75 l lb. 8 oz. Base oil 100 pale make to 55 gal.

The above formulation is a typical mediumtohigh sensitivity penetrant tracer, insoluble in water. It is suitable, e.g., for detecting cracks in weldments, and has a dimensional sensitivity of about 180 millimicrons.

EXAMPLE NO. 3

A penetrant tracer was prepared as follows:

Ethylene glycol monobutyl ether gal 4 Polyoxyethylated nonylphenol (average 45% ethylene oxide) gal 2 Fluoranthene lbs-- 50 C.I. Fluor. Brightening Agent 75 lbs 3 Refined sodium petroleum sulfonate (molecular weight approx. 425) gal-- 13.75 Base oil 100 pale gal. 35.5

The above formulation is a water washable penetrant tracer of high sensitivity, having a dimensional sensitivity of 143 millimicrons. lt is suitable, among other uses, for the detection of micro-llaws in machine parts and ceramic materials.

EXAMPLE NO. 4

A penetrant tracer having the following formulation was prepared:

N-vinyl-2-pyrrolidone gal-- l2 Fluoranthene lbs-- 75 C.I. Fluor. Brightening Agent 75 lbs- 5 Diethylene glycol monobutyl ether gal 4 Polyethylene glycol di-Z-ethylhexoate, to --gal 55 The above formulation is non-water-soluble, and has a dimensional sensitivity of about 75 millimicrons. This tracer is particularly suitable for use in extremely high sensitivity application, in which tests are to be made for intergranular corrosion in metals, or fractures in crystal structures and the like.

EXAMPLE NO. 5

EXAMPLE NO. 6

A penetrant tracer having the following formulation was prepared:

Diethylene glycol monobutyl ether gal 9.2 Fluoranthene lbs-- 25 C.l. Fluor. Brightening Agent 75 lbs-- 5 Polyethylene glycol di-2-ethylhexoate, to gal 55 The above formulation is a high sensitivity, non-watersoluble penetrant tracer having a dimensional sensitivity of about 140 millimicrons. lt is well suited for the penetrant inspection and detection of extremely small surface flaws.

Flux formulations of the invention may be employed in processes for detecting defects in soldered joints. Such processes usually include, as a primary step prior to inspection under black light, the removal of all excess fluorescence-tagged solder flux from the surface of the part t0 Ibe examined. A suitable cleaner is used for this purpose, such as water in the case of water-soluble fluxes, or alcohol in the case of resin-type fluxes. After the removal of the excess surface flux, there will remain minute entrapments of flux in cracks, or in interfacial areas where there is incomplete fusion between solder and base metal. Such small entrapments of the fluorescent flux sometimes can be viewed simply by exposure to black light. However, it may often be found desirable to enhance the fluorescent indications by applying a liquid developer of the aforementioned type to the soldered area. Such a developer will carry the fluorescent flux tracer into solution and allow it to diffuse out into a region where it can become easily seen.

EXAMPLE NO. 7

A soldering flux was prepared as follows:

Glutamic acid hydrochloride 3l lbs. Urea 18 lbs. C.I. Fluor. Brightening Agent 46 27 lbs. C.I. Acid Yellow 7 3 lbs. 8 oz. Polyethylene glycol (molecular weight 200), to 55 gal.

The above soldering flux is of high sensitivity and particularly adapted for use in detecting cold solder joints, or other soldering imperfections. The initial dimensional sensitivity is 75 millimicrons. However, as the flux is heated during the soldering operation, the liquid boils off, leaving interfacial entrapments of substantially dry fluorescent sensitizer material. The film thickness of this residue is quite small, being of the order of a few millimicrons, and the dye concentration is correspondingly higher. Therefore, the ultimate dimensional sensitivity rating for the soldering flux (i.e., as a solid film) is about l0 millimicrons.

EXAMPLE NO. 8

A soldering flux was prepared in accordance with the following formulation:

Water white rosin grams 25 Fluoranthene -do 25 Glutamic acid hydrochloride do 2.5 N-vinyl-Z-pyrrolidone cc 20 Diacetone alcohol cc..- 175 The above soldering flux is of the rosin type and of high sensitivity. It has an initial dimensional sensitivity of about 110 millirnicrons, and an ultimate dimensional sensitivity of about 20 millimicrons.

EXAMPLE NO. 9

A fluorescent trace was prepared as follows:

Percent C.I. Fluor. Brightening Agent 26 15 C.I. Acid Violet 7 (color-former) l Diethylene glycol, to 100.

The above formulation provides an ink suitable for use in industrial marking, ballot marking, or as a business machine ink. It has a dimensional sensitivity of about millimicrons. Such sensitivity level is essential for adequate performance of the ink, since, as may be understood, the latter must reveal its fluorescence even though in the form of an extremely thin fllm. Although a red ink is here provided, it will be realized that other colors, such as yellow or blue, can be obtained through change of the color-former dye.

EXAMPLE No. 1o

A fluorescent tracer concentrate having the following formulation was prepared:

The above concentrate is liquid, and is suitable for use in pools, fountains, or waterfalls, for artistic display effects. When used, the concentrate is mixed with a suitable quantity of Water. For example, pastel fluorescent shades are obtained by a dilution ratio of about 1500 to l, medium shades are obtained by a dilution ratio of 1000 to 1, and deep shades of fluorescent color are obtained by a dilution ratio of 500 to l. The above dye concentrate provides a blue fluorescent color. In this case, the C.I. Fluor. Brightening Agent 57 is used as a combination sensitizer and color-former dye. Other colors may, of course, by provided through the use of different or additional color-former dyes.

EXAMPLE NO. l1

A fluorescent tracer material having the following formulation was prepared:

The above formulation provides a tracer material in dry form, preferred for such uses as the marking of snowed-in airport runways, or as a sea marker for indicating the location of survivors at sea. The powders are blended thoroughly and packaged in moisture-tight containers. The salt is included for the purpose of causing ice or snow to melt so as to allow the fluorescent dyes to go into solution. The silica aerogel serves to render the product fluffy and light, so that it will spread readily over a large area of water, snow, or ice. This formulation exhibits a brilliant fluorescent orange color when in water solution. The color can be changed to a bright red by replacing the fluorescent with eosine, and it can be changed to a brilliant yellow-green by replacing the cosine with fluorescein. When this marker tracer is dissolved in water, it yields a dimensional sensitivity of the order of 100 millimicrons or smaller.

EXAMPLE NO. 12

A fluorescent tracer having the following formulation was prepared:

C.I. Fluor. Brightening Agent 26 oZ.. 8 Water, to make gal 1 The above formulation is a fluorescent blue concentrate having a dimensional sensitivity of about 30 millimicrons, and being suitable for use as a leak tracer for water solutions. When so used, it should usually be diluted in water (e.g., in the proportion of 1 to 200). Its fluorescent color may be shifted to a bright green by the addition of a green color-former, such as C I. Acid Yellow 73 (Fluorescein).

EXAMPLE NO. 13

A crayon having the following formulation was prepared:

Polyethylene glycol (wax) 3lbs. 12oz. C.I. Fluor. Brightening Agent 26 3 oz. C.I. No. 42735-Acid Blue 104 (color former) 3 oz.

White baryt-es 3 lbs.

In forming the above crayon, the polyethylene glycol (wax) was melted and raised to a temperature of about 150 F. The dyes and llers were then added, with the aid of a high speed disperser, and dispersion was continued until Iblending was complete. The hot mixture was then poured into crayon molds. The resulting fluorescent wax had a dimensional sensitivity of about 60 millimicrons. The crayon was found useful for such purposes as marking radar plotting boards, maps, ballots, etc.

EXAMPLE NO. 14

A fluorescent plastic composition for use as a surface coating was prepared having the following percentage composition:

percent Fluoranthene 14 C.I. Fluor. Brightening Agent 75 1.5

Polyester resin (Laminac No. 4110, Trademark,

American Cyanamid Corp.) 84.5

To the above resin composition was added as a catalyst, 8O cc. per gallon of methyl -ethyl ketone peroxide. The mixture was applied in a thin film between two glass plates, and cured to a hard material having a bright green fluorescence, and having a dimensional sensitivity of about millimicrons. A grainless fluorescent screen was thus provided, capable of converting to visible light ultra-violet images focused thereon. This screen may serve as a converter reticle, and is useful in various applications requiring the visual observation of ultra-violet images, e.g., microscopes, or converter cells in communication systems employing modulated ultra-violet radiation.

EXAMPLE NO. 15

A fluorescent tracer having the following composition was prepared:

C.I. Fluor. Brightening Agent 68 gm 120 Ethoxylated nonylphenol (10 mols ethylene oxide per mol nonylphenol) gal 1/2 Water, to make 1 gal.

The above formulation is a fluorescent blue liquid suitable -for marking textiles for cutting or sewing in the manufacture of garments. It is especially useful on dark colored fabrics which require tracers having a high level of sensitivity in order to reveal fluorescence. It has `a dimensional sensitivity of about 200 millimicrons or less.

EXAMPLE NO. 16

A fluorescent tracer having the following composition was prepared:

C.I. Fluor. Brightening Agent 69 grams-- 128 Glycerin, to make 1 gal.

The above formulation has a bright blue fluorescence, and exhibts a dimensional sensitivity onthe order of 200 millimicrons or less. It is particularly useful for application to rubber sealer gaskets, such as refrigerator seals, from which it marks off onto a mating part to show up any discrepancies in the fit of the gasket.

EXAMPLE NO. 17

A fluorescent tracer concentrate having the following composition was prepared:

C.I. Fluor. Brightening Agent 46 lbs 2 Propylene glycol, to make 1 gal.

This is a fluorescent blue material which may be used as an additive to Water soluble soldering fluxes. A proportional concentration is preferred of about 4 oz. of the tracer concentrate per gallon of liquid soldering flux.

When used in this proportion, the tracer concentrate pro vides a dimensional threshold thickness of iluorescence of less than 250 millimicrons, suitable for the detection of unwanted residues of solder ux which may remain after a cleaning operation.

To facilitate the formulation of iiuorescent tracer compositions in accordance with the invention, while allowing for the use of any of a variety of solvents which may be encountered, 4a kit can `be prepared containing an assortment of sensitizer substances, together with an assortment of color-former dyes. The sensitizers and colorformer dyes may be selected so as to provide a good range of typical materials. In using such a kit, it is a relatively simple matter to select a sensitizer which is compatible with a particular solvent system being tested. `It is then quite easy to select a suitable color-former dye, if such is required, to adjust the dye concentrations to their optimum values, and to add any other ingredients, such as thickeners, fillers, or the like.

Although lthe invention has been described with reference to particular embodiments thereof, it will be understood that various changes may Ibe made therein without departing from the spirit of the invention or the scope of the appended claims.

LI claim:

1. :In an inspection process in which thin lms of a uorescen't tracer are revealed by a liuorescence response, the step of preparing a fluorescent tracer by dissolving at least one sensitizer in a solvent carrier, said fluorescent tracer consisting essentially of at least one lsensitizer selected from lthe group consisting of fluorescent (di)amino stilbine(di)sulfonic acid dyes, fluorescent dibenzothiophene dyes, fluorescent monoazole dyes, uorescent bisazole dyes, iiuorescent coumarin dyes, uorescent perylene dyes, lluorescent naphthalic acid imide dyes, uorescent pyridotr-iaZ-ole dyes, fluorescent di-hydrocollidine dyes, pyrene, 1'-hydroxy-Z'-acetonaphthone, ii'uorescent acridine dyes having Color index designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080, Basic Orange 14, 15, 16, yand 23, and Basic Yellow 6 and 9, iluorescent xanthene dyes having Color Index designations of 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45100, 45105, 45155, 45165, 45210, Basic Red 8, and Basic Violet 1l, fluorescent brightening agents having Color Index designations of C. I. Brightening Agent 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61, 65, 66, 67, 68, 69, 70, 7l, 74, 75, 76, 77, 78, and uorescent dyes having Color Index designations of Basic Orange 10, Acid Yellow 7, lBasic Yellow 7, Disperse Yellow 11, 13, tand 31, Direct Yellow 59, Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, Basic Violet 10, Acid Violet 7, Developer 8, and Solvent Red 36 and 45, respectively, and a liquid solvent carrier for said sensitizer, said sensitizer being selectively used singly `and in combination and being present in said liquid solvent carrier Within the range of proportional concentrations from at least about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of uorescence is adjusted to an operational value below about 250 millimicrons.

2. An inspection process in `accordance with claim 1 in which said uorescent tracer consists essentially of at least one sensitizer selected from the group consisting of fluorescent brightening agents having Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57, 68, 69, 75, 78, and dyes having Color Index designations of 49010 and 56205, respectively, and a solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said solvent carrier within the range of proportional concentrations from at least about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of -uorescence is adjusted to an operational value below about 250 millimicrons.

3. An inspection process in accordance with claim 1 in which said lluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of uorescent brightening agents having Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57, 68, 69, 75, 78, and dyes having Color Index designations of 49010 and 56205, respectively, a color-former dye imparting a characteristic color to said tracer, and a solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said solvent carrier within the range of proportional concentrations from at least about 15 grams per pint up to the limit of solubility of said sensitizer, whereby :the dimensional threshold of iluorescence is adjusted to an operational value below about 250 millimicrons.

4. An inspection process in accordance with claim 1 in which said fluorescent tracer consists essentially of at least one .sensitizer 'selected from the group consisting of liuorescent brightening agents having Color Inder designations of 8, 22, 24, 25, 26, 29, 46, 53, 57, 68, 69, 75, 78, and dyes having Color Index desiginations of 49010 and 56205, respectively, and -a liquid solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said liquid solvent carrier within the range of proportional concentrations from at least 'about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

5. An inspection process in accordance with claim 1 in whichsaid fluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of fluorescent brightening agents having Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57, 68, 69, 75, 78, and dyes having Color Index designations of 49010 and 56205, respectively, a color-former dye impanting a characteristic color to said tracer, and a liquid solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said liquid solvent carrier within the range of proportional concentrations from at least about 15 gnams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

6.. An inspection process in accordance with claim 1 in which said uorescent tracer consists essentially of at least one sensitizer selected from the group consisting of fluorescent brightening agents havinge Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57, 68, 69, 75, 78, and dyes having Color Index designations of 49010 land 56205, respectively, and a resinous solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said resinous solvent carrier within the range of proportional concentrations from at least about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

7. An inspection process in accordance with claim 1 in which said fluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of uorescent brightening agents havinge Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57, 68, 69, 75, 78, and dyes having Color Index designations of 49010 and 56205, respectively, a color-forn1er dye imparting a characteristic color to said tracer and a resinous solvent carrier for said sensitizer, said sensitizer beings selectively used singly and in combination and being present in said resinous solvent carrier within the range of proportional concentrations from at least about 15 grams per pint up to the limit of solubility of said sensitizer, where-by the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

8. An inspection process in accordance with claim 1 in which said fluorescent tracer consists essentially of lluoranthene as a sensitizer, and a solvent carrier for said luoranthene, said fluoranthene being present in said solvent carrier within the range or proportional concentrations of at least about 25 grams per pint up to the limit of solubility of said tluoranthene, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

9. An inspection process in accordance with claim 1 in which said fluorescent tracer consists essentially of iluoranthene as a sensitizer, a color-former dye imparting a characteristics color to said tracer, and a solvent carrier for said iluoranthene, said fluoranthene being present in said solvent carrier within the range of proportional concentrations of at least about 25 grams per pint up to the limit of solubility of said iluoranthene, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

10. An inspection process in accordance with claim 1 in which said tluorescent tracer consists essentially of pyrene as a sensitizer, and a solvent carrier for said pyrene, said pyrene being present in said solvent carrier within the range of proportional concentrations of at least about l5 grams per pint up to the limit of solubility of said pyrene, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

11. An inspection process in accordance with claim 1 in which said iluorescent tracer consists essentially of pyrene as a sensitizer, a color-former dye imparting a characteristic color to said tracer, and a solvent carrier for said pyrene, said pyrene being present in said solvent carrier within the range of proportional concentrations of at least about grams per pint up to the limit of solubility of said pyrene, whereby the dimensional threshold of fluorescent is adjusted to an koperation value below about 250 millimicrons.

12. An inspection process in accordance with claim 1 in which said iluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of pyrene; 1-hydroXy2-acetonaphthone, iluorescent acridine dyes having Color Index designations of 46000, and 46025, 46050, 46055, 46060, 46065, Basic Orange 14, 15, 16, 23, and Basic Yellow 6 and 9; fluorescent xanthene dyes having Color Index designations of 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165, 45210, Basic Red 8, and Basic Violet 11; fluorescent brightening agents having Color Index designations of Cl. Brightening Agent 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61, 65, 66, 67, 618, l69, 70, 71, 74, 75, 76, 77, 78; and iluorescent dyes having Color Index designations of Basic Orange 10, Acid Yellow 7, Basic Yellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, Solvent Yellow 44, Solvent Green 5, Acid Red50, 52, and 87, Basic Red 1, Basic Violet 10, Acid Violet 7, Developer 8, and Solvent Red 36 and 45, respectively, and a solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said solvent carrier within the range of proportional concentrations from at least about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

13. An inspection process in accordance with claim l in which said fluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of pyrene; 1-Hydroxy-2-acetonaphthone; fluorescent acridine dyes having Color Index designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080, Basic Orange 14, 15, 16, and 23; and Basic Yellow 6 and 9; lluorescent xanthene dyese having Color Index designations of 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165, 45210, Basic Red 8, and Basic Violet 11; fluorescent brightening agents having Color Index designations of C.I. Brightening Agent 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 3046, 47, 53, 54, 55, 56,

57, 59, 60, 61, 65, 66, 67, 68, 69, 70, 7l, 74, 75, 76, 77, and 78; and iluorescent dyes having Color Index designations of Basic Orange 10, Acid Yellow 7, Basic Yellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, Basic Violet 10, Acid Violet 7, Developer 8, and Solvent Red 36 and 45, respectively, a color-former dye imparting a characteristic color to said tracer, and a solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said solvent carrier within the range of proportional concentrations from atleast about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

1.4. An inspection proceses in accordance with claim 1 in which said iluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of iluoranthene; pyrene; 1-hydroxy 2 acetonaphthone; fluorescent acridine dyes having Color Index designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080, Basic Orange 14, 15, 16, and 23, and Basic Yellow 6 and 9; iluorescent xanthene dyes having Color Index designations of 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165. 45210, Basic Red 8, and Basic Violet 11; lluorescent brightening agents havnig Color Index designations of C I. Brightening Agent 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61, 65, 66, 67, 68, 69, 70, 71, 74, 75, 76, 77, 78; and iluorescent dyes having Color Index designations of Basic Orange 10, Acid Yellow 7, Basic Yellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red l, Basic Violet l0, Acid Violet 7, Developer 8, and Solvent .Red 36 and 45, respectively, and a liquid solvent carrier for said senstizer, said sensitizer being selectively used singly and in combination and being present in said liquid solvent carrier within the range of proportional concentrations from at least about 25 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of lluorescence is adjusted to an operational value below about 250 millimicrons.

15. An inspection process in accordance with claim 1 in which said iluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of fluoranthene; pyrene; 1'-hydroxy-2acetonaphthone; fluorescent acridine dyes having Color Index designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080, Basic Orange 14, 15, 16, and 23, and Basic Yellow 6 and 9; fluorescent xanthene dyes having C-olor Index designations of 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165, 45210, Basic Red 8, and Basic Violet 11; iluorescent brightening agents having Color Index designations of C.I. Brightening Agent 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61, 65, 66, 67, 68, 69, 70, 71, 74, 75, 76, 77, 78; and fluorescent dyes having Color Index designations of Basic Orange 10, Acid Yellow 7, Basic Yellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, Basic Violet 10, Acid Violet 7, Developer 8, and Solvent Red 36 and 45, respectively, a color-former dye imparting a characteristic color to said tracer, and a liquid solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said liquid solvent carrier within the range of proportional concentrations from at least about 25 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of iluorescence is adjusted to an operational value below about 250 millimicrons.

16. An inspection process in accordance with claim 1 in which said iluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of uoranthene; pyrene; l hydroxy-2 .acetonaphthone; liuorescent acridine dyes having Color Index designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080, Basic Orange 14, 15, 16, and 23, and Basic Yellow 6 and 9; fluorescent xanthene dyes having Color Index designations of 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165, 45210, Basic Red 8, and Basic Violet 11; iluorescent brightening Agent 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 53, 54, 55, 56, 57, 59, 60, 61, 65, 66, 67, 68, 69, 70, 71, 74, 75, 77, 78; and uorescent dyes having Color Index designations of Basic Orange 10, Acid Yellow 7, Basic Yellow 7, Disperse Yellow 1l, 13, and 31, Direct Yellow 59, Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, Basic Violet l0, Acid Violet 7, Developer 8, and Solvent Red 36 and 45, respectively, and resinous solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said resinous solvent carrier wit-hin the range of proportional concentration from at least about grams per pint up to the limit of solubility tof said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

17. An inspection process in accordance with claim 1 in which said iluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of iluoranthene; pyrene; 1 hydroxy 2 acetonaphthone; fluorescent acridine dyes having Color Index designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080, Basic Orange 14, 15, 16 and 23, and Basic Yellow 6` and 9; fluorescent xanthene dyes having Color Index designations of 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165, 45210, Basic Red 8, and Basic Violet l1; uorescent brightening agents having Color Index designations of C.I. VBrightening Agent 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61, 65, 66, 67, 68, 69, 70, 71, 74, 75, 76, 77, 78; and fluorescent dyes having Color Index designations of Basic Orange 10, Acid Yellow 7, Basic Yellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, Basic Violet 10, Acid Violet 7, Developer 8, and Solvent Red 36 and 45, respectively, a color-former dye imparting a characteristic color to said tracer, and a resinous solvent carrier for said sensitizer, said sensitizer, being selectively used singly and in combination and being present in said resinous solvent carrier within the range of proportional concentrations from at least about 25 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

18. An inspection process in accordance with claim 1 in which said fluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of fluorescent (di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophene dyes, iluorescent monoazole dyes, fluorescent bisazole dyes, fluorescent coumarin dyes, uorescent perylene dyes, uorescent naphthalic acid imide dyes, fluorescent pyridotriazole dyes, and iluorescent di-hydrocollidine dyes, respectively, and a solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said solvent carrier within the range of proportional concentrations of from at least about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

19. An inspection process in accordance with claim 1 in which said uorescent tracer consists essentially of at least one sensitizer selected from the group consisting of iluorescent (di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophene dyes, uorescent monoazole dyes, fluorescent bisaZQle dyes, uorescent 4coumarin dyes,

fluorescent perylene dyes, liuorescent naphthalic acid imide dyes, fluorescent pyridotriazole dyes, and fluorescent di-hydrocollidine dyes, respectively, a color-former dye imparting a characteristic color to said tracer, and a solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said solvent carrier within the range of proportional concentrations of from at least about 15 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value -below about 250 millimicrons.

20. An inspection process in accordance with claim 1 in which said iluorescent tracer consists essentially at least one sensitizer selected from the group consisting of fluorescent (di)aminostilbine(di)sulfonic acid dyes, uorescent dibenzothiophene dyes, fluorescent monoazole dyes, uorescent bisazole dyes, iluorescent coumarin dyes, fluorescent p-erylene dyes, fluorescent naphthalic acid imide dyes, fluorescent pyridotriazole dyes, and uorescent di-hydrocollidine dyes, respectively, and a liquid solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said liquid solvent carrier within the range of proportional concentrations of from at least about 25 grams per -pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value lbelow about 250 millimicrons.

21. An inspection process in accordance with claim 1 in which said fluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of luorescent (di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophene dyes, uorescent monoazole dyes, uorescent bisazole dyes, lluorescent coumarin dyes, :liuorescent perylene dyes, iluorescent naphthalic acid imide dyes, uorescent pyridotriazole dyes, and uorescent di-hydrocollidine dyes, respectively, a color-former dye imparting a characteristic color to said tracer, and a liquid solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said liquid solvent carrier within the range of proportional concentrations of from at least about 25 `grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

22. An inspection process in accordance with claim 1 in which said Iiluorescent tracer consists essentially of at least one sensitizer selected from the group consisting of fluorescent (di)aminostilbine(di)sulfonic acid dyes, uorescent dibenzothiophene dyes, fluorescent monoazole dyes, fluorescent bisazole dyes, fluorescent coumarin dyes, fluorescent perylene dyes, iluorescent naphthalic acid imide dyes, uorescent pyridotriazole dyes, and fluorescent di-hydrocollidine dyes, respectively, and a resinous solvent carrier for said sensitizer, said sensitizer being selectively used singly and in combination and being present in said resinous solvent carrier within the range of proportional concentrations of from at least about 25 grams per pint up to the limit of solubility of said sensitizer, whereby the dimensional threshold of fluorescence is adjusted to an operational value below about 250 millimicrons.

23. An inspection process in accordance with claim 1 in which said uorescent tracer consists essentially of at least one sensitizer selected from the group consisting of uorescent (di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophene dyes, fluorescent monoazole dyes, fluorescent bisazole dyes, fluorescent coumarin dyes, fluorescent perylene dyes, fluorescent naphthalic acid imide dyes, fluorescent pyridotriazole dyes, and uorescent di-hydrocollidine dyes, respectively, a color-former dye imparting a characteristic color to said tracer, and a resinous solvent carrier for said sensitizer, said sensitizer 21 being selectively used singly and in combination and being present in said resinous solvent carrier within the range of proportional concentrations of from at least about 25 -grams per pint to the limit of solubility of said sensitizer, whereby the dimensional threshold of uorescence is adjusted to an operational valve below about 250 millimicrons.

References Cited UNITED STATES PATENTS 2,733,216 1/1956 Eichholz et al 252-3012 2,953,530 9/1960 Switzer 252-3012 3,108,187 10/1963 Thornbury 252-3012 FOREIGN PATENTS 569,920 6/ 1945 Great Britain. 143,492 8/1961 U.S.S.R.

5 OTHER REFERENCES Dement: Fluorochemistry, 1945, lpages 160, 187, 194 and 198.

TOBIAS E. LEVOW, Primary Examiner.

lo ROBERT D. EDMONDS, HELEN M. MCCARTHY,

Examiners.

UNITED STATES PATENT oFEICE CERTIFICATE OF CORRECTION Patent No. 3,386,920 June 4, 1968 James R. Alburger It is Certified that error appears in the above identified patent and that said Letters Patent are hereby Corrected as shown below:

Column 3, lino l5, "direction should read direct line 68, "the" should read that Column 4, line l0, cancel the letter "l" and insert the numeral l Column 9, line Z8. Cancel "now abandoned". Column l0,

line I6, "of" should read or Signed and Sealed this 7th day of October 196g.

(SEAL) Attest:

WTILLIAM E. SCHUYLER, JR-

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. IN AN INSPECTION PROCESS IN WHICH THIN FILMS OF A FLUORESCENT TRACER ARE REVEALED BY A FLUORESCENCE RESPONSE, THE STEP OF PREPARING A FLUORESCENT TRACER BY DISSOLVING AT LEAST ONE SENSITIZER IN A SOLVENT CARRIER, SAID FLUORESCENT TRACER CONSISTING ESSENTIALLY OF AT LEAST ONE SENSITIZER SELECTED FROM THE GROUP CONSISTING OF FLUORESCENT (DI)AMINOSTILBENE(DI)SULFONIC ACID DYES, FLUORESCENT DIBENZOTHIOPHENE DYES, FLUORESCENT MONOAZOLE DYES, FLUORESCENT BISAZOLE DYES, FLUORESCENT COUMARIN DYES, FLUORESCENT PERYLENE DYES, FLUORESCENT NAPHTHALIC ACID IMIDE DYES, FLUORESCENT PYRIDOTRIAZOLE DYES, FLUORESCENT DI-HYDROCOLLIDINE DYES, PYRENE, 1''-HYDROXY-2''-ACETONAPHTHONE, FLUORESCENT ACRIDINE DYES HAVING COLOR INDEX DESIGNATIONS OF 46000, 46025, 46050, 46055, 46060, 46065, 46080, BASIC ORANGE 14, 15, 16, AND 23, AND BASIC YELLOW 6 AND 9, FLUORESCENT XANTHENE DYES HAVING COLOR INDEX DESIGNATIONS OF 45000, 45005, 45006, 45010, 45015, 45020, 45050, 45090, 45100, 45105, 45155, 45165, 45210, BASIC RED 8, AND BASIC VIOLET 11, FLUORESCENT BRIGHTENING AGENTS HAVING COLOR INDEX DESIGNATIONS OF C. I. BRIGHTENING AGENT 2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61, 65, 66, 67, 68, 69, 70, 71, 74, 75, 76, 77, 78, AND FLUORESCENT DYES HAVING COLOR INDEX DESIGNATIONS OF BASIC ORANGE 10, ACID YELLOW 7, BASIC YELLOW 7, DISPERSE YELLOW 11, 13, AND 31, DIRECT YELLOW 59, SOLVENT YELLOW 44, SOLVENT GREEN 5, ACID RED 50, 52, AND 87, BASIC RED 1, BASIC VIOLET 10, ACID VIOLET 7, DEVELOPER 8, AND SOLVENT RED 36 AND 45, RESPECTIVELY, AND A LIQUID SOLVENT CARRIER FOR SAID SENSITIZER, SAID SENSITIZER BEING SELECTIVELY USED SINGLY AND IN COMBINATION AND BEING PRESENT IN SAID LIQUID SOLVENT CARRIER WITHIN THE RANGE OF PROPORTIONAL CONCENTRATIONS FROM AT LEAST ABOUT 15 GRAMS PER PINT UP TO THE LIMIT OF SOLUBILITY OF SAID SENSITIZER, WHEREBY THE DIMENSIONAL THRESHOLD OF FLUORESCENCE IS ADJUSTED TO AN OPERATIONAL VALUE BELOW ABOUT 250 MILLIMICRONS. 